Underground mining of mineral ores, such as coal and hard and soft rock mining requires the ‘development’ of underground drives in the form of tunnels. In all hard-rock applications, drive development is achieved through a drilling, charging, blasting, and mucking cycle. In the drilling stage of the cycle, a pattern of holes is drilled into the blind end of the drive. The holes are generally parallel to the drive axis. Typically, holes are 2–4 meters deep.
In the charging stage, explosive is placed in the drilled holes and connected via a detonating arrangement. In the blasting stage the explosive is detonated, the resulting blast fracturing the solid rock. In the mucking stage a front-end loader digs the fractured rock and removes it for hoisting to the surface via skips. This development cycle is well understood and is currently the most cost effective means of developing drives in hard rock.
An unavoidable consequence of this proven method is rock fracture beyond the desired geometric shape of the tunnel cross-section. This rock fracturing can cause the tunnel roof or back and/or the drive's side-walls to be unstable. Rock fragments large and small can disengage from the back and sidewalls and fall under the influence of gravity. Particle size ranges from microscopic to cubic meters. Falling particles larger than a tennis ball can prove fatal to personnel.
To protect miners from larger falling particles, a rock bolting/meshing procedure is applied. The process requires drilling holes 2–4 meters long in the ‘back’ (walls and overhead), and holding square mesh, typically 50 mm×50 mm to 150 mm×150 mm apertures, against the ‘back’. Rock bolts and retaining plates are inserted through the mesh and into the drilled holes. Larger particles are restrained from falling by the rock-bolts and smaller particles are retained or caught by the mesh.
Rock bolts come in various styles and each style is available in a range of lengths. Common styles include the split set type where long slotted tubes grip the drilled hole via radial springing action along the entire length of the bolt. These bolts rust away in time and jeopardize long-term security. The wedge-lock type is a bolt with an expanding tip, the locking action being controlled by screwing action. The gripping is at the blind end of the hole only. These bolts also rust away in time and jeopardize long-term security. Epoxy grouted systems utilize a two-pack epoxy sausage which is inserted into the drilled hole. The bolt is inserted via a rotating action that mixes the epoxy. Curing is rapid usually taking about 35–60 seconds. In such epoxy grouted systems gripping occurs substantially along the entire length of the bolt/hole. These epoxy grout system bolts resist corrosion. Cement grouted systems are also used.
Rock-bolting/meshing equipment comes in two broad groups, comprising purpose built drilling bolting machines and adaptations of twin boom development heading ‘jumbo’ drills. The purpose built drilling bolting machines generally feature three parts, being a transport vehicle subassembly, a multi-axis support arm mounted thereon and a drilling and bolting mechanism on the support arm. The drilling and bolting mechanism contains many functions and is relatively heavy, both for robustness and to provide inertial stability. The multi-axis support arm, while capable of supporting the mechanism, tends to deflect, has low natural frequencies of bobbing up/down and back/forth and also has poor ‘fine control’. The transport vehicle is rubber tyred, with articulated steering, diesel powered and with front jacks for vehicle stability while working.
In use, problems arise because of the physical properties of the freshly fractured rock surface. It is uneven and fractured, presenting a myriad of randomly oriented faces. Lighting from the vehicle throws this surface into stark black/white features where the operator cannot determine the inclination of faces to select a stable face for drilling.
Collaring is the step of the drill taking purchase and commencing the new hole and usually describes the first 0–20 mm of drilling. The drill head is a blunt steel arrangement with embedded tungsten carbide tips, air or water cooled and purged via a central hole along the drill steel. Cutting is by rotation and impact from the drill, with typical drilling speeds being at 1–2 meters per minute. When the blunt drill head strikes an angled rock face in attempting to collar a new hole, it generally cannot achieve penetration. Instead the drill slides down the face until it finds purchase in the ‘valley’ between two intersecting planes of the rock faces. Collaring now proceeds as does the remainder of the hole drilling.
The drill bit, sliding down the rock face and into the ‘valley’ demands lateral compliance since the support arm's hydraulics have not yielded or adjusted. Compliance is available from many sources including elastic bending of the drill steel, mechanical play or hackles in the drill steel/drill interface, the drill/drill slide interface and every other mechanical junction, deflection in the supporting arm, and deflection in the supporting vehicle.
The drill achieves a collared and drilled hole, albeit not precisely where the drill was aimed. Upon drill steel extraction form the new hole, the elastic compliance is released and the whole machine wobbles back and forth, finally settling with the drill steel axis no longer aligned with the freshly drilled hole. The mechanism now increments, removing the drill from the axis and replacing it with a bolt magazine with an inserted bolt. The bolt has little chance of finding the hole because the mechanical ‘slop’ (play, clearance, backlash) is endemic, with machine parts which are expected to operate reliably despite spending their lives in a shower of water, grit and falling rocks. The net effect is that the drilled hole will often not be co-axial with the bolt. Rock fragments often fall from the ‘back’ around the freshly drilled hole to sit on the mesh, masking the hole. Attempting to insert an all metal bolt is normally unsuccessful.
The machine operator then gets out of his protected cabin and walks under the unprotected cabin and walks under the unprotected, freshly fractured, freshly drilled ground to try and find the offset error between where the hole axis lies and where the bolt axis lies. This is the most dangerous time with a high risk of falling rock causing death or injury. The operator then goes back to his machine and tries to remember the direction and distance of the offset and, using an arm with poor ‘fine control’, attempts to adjust for the error. There are often several attempts required to adjust for bolt insertion. With epoxy-grouted bolts, these aiming problems can see the two-part epoxy sausage bursting, covering the drilled/bolting mechanism and/or the hole opening with rapidly setting epoxy, which can disable the mechanism and/or block the hole.